Students now build on the “same”type and “opposite”type constructions, as they learn to combine two mechanisms into one. They create pegboard mechanisms that feature a common input controlling two outputs. They compare the directions of motion of the two outputs, and learn how a system can be built up from two simpler subsystems.
In a class meeting: Demonstrate two links joined with a pivot. Ask:
Distribute a base, three links and pivots to each student and present this challenge:
Make a mechanism with one input that controls two outputs.
After students have made a mechanism they draw the mechanism in their science notebooks.
Whole-class discussion: Select students to present their mechanisms so that each of the types shown on the Combining Mechanisms Worksheet is represented. (See a video on four mechanisms) For each mechanism, draw the base and links on chart paper. Ask students where to place the pivots and how to draw them. (See a video on drawings of four mechanisms.) Ask how to show the directions of motion. Put the drawings of the four types of mechanisms side by side. Conduct a discussion about why each of the outputs moves as it does. Ask:
Systems and sub-systems.
Explain that each mechanism with one input and two outputs is a complex system. This complex system is made from two simple systems and that each simple system has one input and one output. The simple systems are sub-systems of the two output system. Here is a video on these systems and subsystems.
The simple lever systems “Same” and “Opposite” can be combined to make complex systems where, relative to the direction of the input, the two outputs go “Same – Same”, “Opposite – Opposite”, “Same – Opposite” and “Opposite – Same”.
Students should discover that:
* To make two outputs move in the same direction, their fixed pivots must be on the same side of the input link, either both above the input (between the floating pivot and the output) or both below the input link .
* To make two outputs move in opposite directions, their fixed pivots must lie on opposite sides of the input link.
Complete the Combining Mechanisms Worksheet and place in the science notebook.